1. Field of the Invention
The present invention relates to a method for manufacturing single crystals by the Czochralski Method (CZ method), particularly to a method and a apparatus for manufacturing single crystals which can reduce the oxygen density along longitudinal axes of produced single crystals.
2. Description of the Prior Art
Single crystals are conventionally produced by the CZ method. In the CZ method, polycrystalline silicon lumps are fed into a quartz crucible of a single crystal manufacturing apparatus, then the quartz crucible is heated by heaters disposed therearound to melt the polycrystalline silicon lumps, thereafter a seed crystal installed on a seed chuck is dipped into the melt. After that, the seed chuck and the quartz crucible are respectively driven to rotate in opposite or identical directions, and at the same time the seed chuck is pulled up to grow a single-crystal silicon ingot of predetermined diameter and length.
Due to the fact that the inner surface of the crucible is in contact with the melt, oxygen contained in the inner surface of the crucible dissolves into the melt. Most of the oxygen evaporates through the surface of the melt and is expelled out of the single crystal manufacturing apparatus, however, the remaining oxygen enters the single crystal being pulled up. The density of oxygen in single crystals is high at the beginning of the pulling up operation and declines following an increase of the solidification ratio. The oxygen entering the single crystal plays the role of cleaning up contamination by a very small amount of heavy metal during the manufacturing process of semiconductor devices, in other words, if exhibits an intrinsic-gettering effect. However, when oxygen remains in the active layers of wafer surfaces, it becomes nuclei of oxidation-induced stacking faults and thus negatively affects the electric characteristics of semiconductor devices.
To equalize oxygen density along the longitudinal axis of a single-crystal ingot, various proposals are recommended. For example, a method for manufacturing single crystals is disclosed in the Japanese publication gazette TOKU KAIHEI 2-192486, in which the output of heaters disposed around the body and the bottom of a quartz crucible is controlled. Also, a method for manufacturing single crystals is disclosed in TOKU KAIHEI 2-217388, in which a quartz crucible is provided with heat shield members used for covering all of the quartz crucible except for the single-crystal silicon pulling up zone, during the operation of pulling up single-crystal silicon by the CZ method, and the output of heaters disposed around the body and the bottom of the quartz crucible are controlled. Furthermore, in TOKU KAIHEI 2-229786, a horizontal magnetic field is applied to the melt in a quartz crucible and the output of heaters disposed around the body and the bottom of the quartz crucible is controlled.
Also, in TOKU KAI HEI 5-294782, the oxygen density of single-crystal silicon is reduced by controlling the melt temperature via a main heater and a multiple staged sub-heater disposed above the melt and shaped in concentric circles.
However, in recent years, as the volume of quartz crucibles rises following the enlargement of the sizes of single crystal silicon ingots, both an electric amount of power required by heaters and surface of quartz crucibles in contact with melt are increased, and then the amount of oxygen coming from quartz crucibles and dissolved in melt has also increased. Hence, it is difficult to obtain low oxygen single crystals with oxygen density of less than 12.times.10.sup.17 atoms/cm.sup.3 or extreme low oxygen single crystals with oxygen density of less than 10.times.10.sup.17 atoms/cm.sup.3 by using above conventional methods. In addition, it takes a long time to grow large single-crystal silicon, therefore there exists a danger of worsening quality of quartz crucibles by heat load and polycrystallizing single crystals during the pulling up operation. In view of the above situation, it is difficult to impose minimum heat load on quartz crucibles during pulling up operation as conventionally. Furthermore, in TOKU KAI HEI 6-183876, the inventors of this invention disclosed a method and an apparatus suitable for recharging material when single-crystal silicon has been pulled up, or for applying a supplementary charge of material when a preset amount of material has been melted and additional material is required to increase the amount of melt.